The present invention relates to an induction motor system having an induction motor and a controller that employ voltage modulation to control the motor speed. More particularly, the present invention relates to such an induction motor system for use in a wet or electrically hazardous environment.
Bathing appliances such as hot tubs and hydromassage bath fixtures are preferably designed such that the strength of water flow is adjustable, as some bathers prefer more forceful water flow than others. The flow and pressure characteristics of the centrifugal pumps used to pump the water should therefore be adjustable by the bather. It is known to control the flow and pressure characteristics of the water through the use of mechanically or electrically operated restrictors and diverter valves. Alternatively, the speed of the electric motor may be controlled.
Variable speed electric motors and controllers therefor are available and well known in the art. Known controllers generally employ mechanical speed control means, inverters, or involve voltage control of universal motors.
Mechanical speed control means typically employ gearing systems that reduce or increase the output speed of a motor shaft. Such mechanical gearing systems may be adjustable to allow the motor to be operated at various speeds.
Prior art controllers are also known that utilize an inverter to convert a DC input into a fixed step current wave form that is applied to the motor. In many cases, the cost of the inverter is greater than the cost of the motor and associated equipment, and therefore inverter technology is used primarily in high end applications. Inverter methods are exemplified in U.S. patent application Ser. No. 08/291,239 (Kemp) entitled Voltage Phase and Frequency Control by Miniature Inverter System, and in U.S. Pat. No. 4,465,961 (Landino) entitled Motor Control System.
Prior art voltage control methods vary the speed of an electric motor (typically a brush-type universal motor) by varying the AC input. The speed variation is accomplished either by varying the voltage or the phase angle of the AC input that drives the motor. Although the cost of a universal motor and a variable speed controller therefor is relatively low, the motor must rotate at a high rate of speed. Such units are therefore generally noisy and have a low life expectancy due to brush wear on the commutator shaft of the motor.
Another known system includes a three speed induction motor utilizing a standard tapped main winding to produce 3600 rpm and two alternate speeds on a two/four pole winding which produces 3600 rpm and 1800 rpm respectively. Such systems generally have limited functionality or excessive cost for the features provided.
Alternatively, an AC induction motor coupled to a variable speed motor controller may be used to drive the water pump of a hydromassage bath. Such units are effective, reliable, inexpensive, long lasting and reasonably quiet. An effective way of varying the speed of such a motor is to provide a motor controller for varying the applied voltage to the motor; for example, by inducing a phase angle delay. The motor may thus be made to reduce speed from its synchronous speed by lowering applied voltage. Lowering the applied voltage reduces the power density in the stator windings, thus reducing the magnetic flux generation in the stator windings. The lower magnetic flux reduces the torque applied to the spinning rotor, thereby reducing the speed of the motor.
An AC induction motor with a voltage modulation motor controller to control motor speed is an effective arrangement for providing an adjustable flow of water to a bather in a hydromassage bath. However, the power necessary to operate a hydromassage AC induction motor pump is sufficiently great to create a shock hazard to bathers operating controls while in the bath. Thus, the bather will be in danger of electrocution if not protected from the electric source of the motor while operating the controls to vary the flow of water. This creates a practical dilemma as the user cannot convey his commands to the pump control without xe2x80x9cbridgingxe2x80x9d the electrical isolation barrier, putting himself at risk of shock.
Accordingly, it is an object of the present invention to provide a motor system having a variable speed AC induction motor and motor controller for driving a pump for a hydromassage bath.
It is a further object of the present invention to provide a motor system having a variable speed AC induction motor and motor controller safely operable by a bather immersed in water.
To protect the bather from electric shock, the electrical energy driving the pump is sufficiently isolated from the bather by isolating the control circuits from the electrically conductive bath water.
According to an aspect of the invention, there is provided an apparatus operable in a wet environment for controlling the speed of an induction motor adapted to be coupled to an AC source for supplying an AC signal, comprising:
a switching device coupled to the motor, the switching device being operative in either a first state wherein significant current flow through the motor is prevented or a second state wherein current flow through the motor is substantially undisturbed;
user controls for providing motor speed input signals;
controller means for receiving the motor speed input signals from the user controls, and for switching the switching device between its first and second states in a predetermined sequence for inducing a phase angle delay in the AC signal; and
isolation means for electrically isolating the user controls from the AC source, wherein the isolation means includes coupling means for optically coupling the switching device to the controller means.
According to another aspect of the invention, there is provided an apparatus operable in a wet environment for controlling the speed of an induction motor adapted to be coupled to an AC source for supplying an AC signal, comprising:
a switching device connected in series with the motor, the switching device being operative in either a high impedance state wherein significant current flow through the motor is prevented or a low impedance state wherein current flow through the motor is substantially undisturbed;
user controls for providing motor speed input signals;
controller means for receiving the motor speed input signals from the user controls, and for switching the switching device between its low and high impedance states in a predetermined sequence for inducing a phase angle delay in the AC signal; and
isolation means for electrically isolating the controller means from the AC source, wherein the isolation means includes coupling means for optically coupling the switching device to the controller means.
According to another aspect of the invention, there is further provided a method for controlling the speed of an induction motor in a wet environment adapted to be coupled to an AC source for supplying an AC signal, and comprising:
a switching device coupled to the motor, the switching device being operative in either a first state wherein significant current flow through the motor is prevented or a second state wherein current flow through the motor is substantially undisturbed;
user controls for providing motor speed input signals;
controller means for receiving the motor speed input signals from the user controls, and for switching the switching device between its first and second states in a predetermined sequence for inducing a phase angle delay in the AC signal; and
isolation means for electrically isolating the user controls from the AC source, wherein the isolation means includes coupling means for optically coupling the switching device to the controller means;
the method comprising the steps of:
(a) at a zero-crossing point of the positive half cycle of the AC signal, switching the switching device to its first state;
(b) generating a first delay;
(c) switching the switching device to its second state;
(d) at a zero-crossing point of the negative half cycle of the AC signal, switching the switching device to its first state;
(e) generating a second delay;
(f) switching the switching device to its second state.
According to another aspect of the invention, there is further provided a method for controlling the speed of an induction motor adapted to be coupled to an AC source for supplying an AC signal, and comprising:
a switching device connected in series with the motor, the switching device being operative in either a high impedance state wherein significant current flow through the motor is prevented or a low impedance state wherein current flow through the motor is substantially undisturbed;
user controls for providing motor speed input signals;
controller means for receiving the motor speed input signals from the user controls, and for switching the switching device between its low and high impedance states in a predetermined sequence for inducing a phase angle delay in the AC signal; and
isolation means for electrically isolating the controller means from the AC source, wherein the isolation means includes coupling means for optically coupling the switching device to the controller means;
the method comprising the steps of:
(a) at a zero-crossing point of the positive half cycle of the AC signal, switching the switching device to its high impedance state;
(b) generating a first delay;
(c) switching the switching device to its low impedance state;
(d) at a zero-crossing point of the negative half cycle of the AC signal, switching the switching device to its high impedance state;
(e) generating a second delay;
(f) switching the switching device to its low impedance state.
According to another aspect of the present invention, there is further provided an induction motor adapted to be coupled to an AC source and a motor controlling apparatus for controlling the motor speed by varying voltage phases supplied to the motor from the AC source, the motor comprising:
a motor drive shaft;
a rotor mounted on the motor drive shaft, the rotor having rotor laminations on its outer surface, the rotor laminations having a plurality of conductor bar slots and having a reflected rotor resistance over reflected rotor reactance ratio that is greater than one;
conductor bars accommodated in the conductor bar slots;
end rings for short-circuiting the conductor bars to form a rotor winding;
stator windings adapted for providing rotational forces to the rotor in cooperation with the rotor winding; and
a stator for supporting the stator windings.
According to yet another aspect of the present invention, there is further provided an induction motor system operable in a wet environment, the system comprising: an induction motor adapted to be coupled to an AC source for supplying an AC signal, the motor having:
a motor drive shaft;
a rotor mounted on the motor drive shaft, the rotor having rotor laminations on its outer surface, the rotor laminations having a plurality of conductor bar slots and having a reflected rotor resistance over reflected rotor reactance ratio that is greater than one;
conductor bars accommodated in the conductor bar slots;
end rings for short-circuiting the conductor bars to form a rotor winding;
stator windings adapted for providing rotational forces to the rotor in cooperation with the rotor winding; and
a stator for supporting the stator windings; and
a motor controlling apparatus for controlling the speed of the motor, the controlling apparatus having:
a switching device coupled to the motor, the switching device being operative in either a first state wherein significant current flow through the motor is prevented or a second state wherein current flow through the motor is substantially undisturbed;
user controls for providing motor speed input signals;
controller means for receiving the motor speed input signals from the user controls, and for switching the switching device between its first and second states in a predetermined sequence for inducing a phase angle delay in the AC signal; and
isolation means for electrically isolating the user controls from the AC source, the isolation means including coupling means for optically coupling the switching device to the controller means.